WO 2004/051303 A1 discloses a method and a system for monitoring and position determination of objects and/or living beings within an area, such as, e.g. a room in a building or a road tunnel. The system comprises a plurality of identification tags equipped with an ultrasonic receiver and radio transmitter which is attached to the objects that have to be monitored. The identification tags receive ultrasonic signals whose arrival time they measure. This information together with the identification tags' ID code are transmitted by radio waves to a central unit which calculates the position of each of the identification tags.
It is a disadvantage of the known system that the processing that must be carried out at the identification tags is relatively complex, and that a large amount of data may have to be transmitted from the beacons to the tags, which in turn leads to a high energy consumption at the tags. It is an object of embodiments of the present invention to at least partially overcome these disadvantages.